he strata according to time, but, as Sandberger says, in quite a
varied mixture, yet in all imaginable modifications. But even among the
higher and the highest classes of animals, we can trace the transitions.
The flying sauria, if not in their organs of flying, which remind us more
of the bat, at least in head, neck, and toes, are closely connected with
the {83} birds--the oldest birds of the Jura and chalk formations, with
their tail-spines similar to the reptilia and their teeth in the beak to
the sauria. The tertiary formations especially show the primitive history
of many vertebrates in very instructive forms of transitions--which, for
instance, Ruetimeyer, a scientist who is very cautious in his conclusions,
very distinctly traced to the horse, to the ruminating animals, and lately
also to the turtles. Still more in detail, W. Kowalewsky has lately shown
us the primitive history of the horse, and Leidy and Marsh have further
completed it by the addition of American forms, the former having at the
same time described the forms which have led to the tapir.

But to such facts there are, on the other hand, experiences directly
contradictory. Many lower and higher forms of animals and plants appear in
the geological strata, so far as they have been explored, in a wholly
independent way. We have mentioned, in the foregoing section, that the main
types of the invertebrates appear somewhat contemporaneously and without
any traceable intermediate form. The trilobites, a quite highly organized
order of crustacea, appear in the strata of the silurian epoch almost
suddenly, in very many and very distinctly marked species. The uncertainty
of our knowledge shows itself most clearly when we ask for the geneologic
relationship of the vertebrates. In Chap. II, Sec. 1 and Sec. 2 we have already
referred to the value which Darwin, and more especially Haeckel, lays on the
relationship of the larva of the ascidia to the lancelet fish. Now the
important testimony of K. E. von Baer, in his "Memoires de l'Academie de
St. Petersbourg," Ser. vii, Vol. 19, No. 2, tells us that the
nerve-ganglion {84} of the ascidia lies on the side of the stomach, and on
that account can not be homologous with the spine of the vertebrates, but
that the cord in the larva of the ascidia is nothing more than a support
for the tail in swimming, which afterwards disappears, as with many other
larvae. As to the course of reasoning in reaching these genealogical
con